A: "Biologically men and women are different, and therefore it shouldn't surprise they have different salary on average".
B: "That is obvious. But you cannot derive from such a difference in terms of biology a difference in regulation terms".
B has clearly made a fallacy: s/he answered to a point that was not antecedently made by A, given the impression he did correct A on something. Is correct to refer to this fallacy as ignoratio elenchi or there is a more specific name for it?
I don't think that B has made a fallacy. What A is really saying (imho) is this:
(1)If Biologically men and women are different then it shouldn't surprise they have different salary on average.
It is a conditional. Additionally, A is also stating that the antecedent is true.
(2) Biologically men and women are different.
Now what B does is to say that while (2) is true, (1) is not true.
One may construe A's claim as a claim about causation…
The fact that men and women have different salary on average is caused by the fact that men and women are biologically different
… but the sense in which the point is phrased is more akin to an explanation:
The fact that men and women are biologically different (sufficiently) explains the fact that men and women have different salaries on average.
In your scenario B replies:
That is obvious. But you cannot derive from such a difference in terms of biology a difference in regulation terms.
Now, what does B's statement "That is obvious" refer to? From the overall scenario I'd think that he agrees with A that both claims – (1) men and women are biologically different and (2) men and women have different salaries on average – are true as a matter of fact. What is she disagreeing with? Your scenario leaves two options open:
She may disagree that (1) is an explanation of (2).
This is not a fallacy at all. B draws attention to the fact that one can agree that both (1) and (2) are true and still disagree that there's any relevant explanatory connection between them.
She may agree or disagree with B that (1) is an explanation of (2). What she objects to is the circumstance that A may be implying to derive a justification. Clearly, even if (1) was a valid explanation of (2), one could not derive any easy normative justification for it.
Whether this is an unwarranted argumentative move (in which case we would speak of a straw man argument) or not completely depends upon the question whether B is right to attribute to A to have made such an implication.
(Personally, if someone made a claim like A's, I would rather take the risk to misrepresent A's position than let the possible implication stay in the room.)
In contrast to the other answers, I break down Person A and Person B's arguments differently.
Person A it seems to me makes the following argument:
1) If two individuals have different attributes and functions, they produce different results in a career.
2) These different results can and often do result in different salaries.
3) Biologically, men and women are different in attributes and functions.
4) Therefore, it should not be surprising that men and women have different salaries on average.
Person A makes no normative judgement as to whether this should be the case.
Person B makes the following argument in response:
1) It is obvious that men and women's biological differences produce different results in a career.
2) This situation does not have to be resolved by allowing the course of action to flow to a difference in salary.
3) Regulations can be put in place that control the environment to produce equivalent outcomes for both male and female employees.
4) You cannot derive a normative requirment for men and women to have different salaries if this regulation is possible.
And this last point (number 4) in Person B's argument is where there is a fallacy. It is a straw man argument because it is a claim that Person A made a normative judgment where that person did not.
